The present invention relates to a method for producing silicon nitride molded bodies by means of a pseudoisostatic hot pressing process wherein the mold of the hot pressing furnace is carbon. The present invention also relates to apparatus for practicing the method.
The ceramic substance Si.sub.3 N.sub.4 has been examined for years as to its suitability as a material for fabricating high temperature combustion gas turbines. The requirements for use in such turbines include good hardness and thermal shock resistance over a temperature range from room temperature to about 1300.degree. C.
In the manufacture of Si.sub.3 N.sub.4 molded bodies, two significantly different methods can be employed. The first method relates to the nitriding of porous silicon blanks (reaction bounded silicon nitride) which results in the formation of a silicon nitride product that is a porous body, exhibiting decreased hardness, but relatively satisfactory high temperature (HT) properties. The other method relates to the hot pressure sintering of blanks made from .alpha.-Si.sub.2 N.sub.4 powder (i.e., hot pressed silicon nitride) which results in the formation of a product which, when the quantity of sinter-enhancing additives is sufficient, has a density corresponding to its theoretical density and thus possesses good hardness values. The high temperature properties are sometimes substantially worsened by incorporating the sintering additives, e.g., MgO, into the sintering mixture because the sintering additives do not become incorporated into the lattice structure of the Si.sub.3 N.sub.4. Thus, under sintering conditions, MgO forms a liquid phase with SiO.sub.2 which is always present at the surface of the Si.sub.3 N.sub.4 particles, (which is the reason for the sinter-enhancing effect) and this liquid phase hardens amorphously at the grain boundaries. This vitreous phase, which contains intergranular silicate, softens at relatively low temperatures and thereby causes an undesirable high temperature plasticity of the resulting product.
Other sinter-enhancing additives, e.g., Al.sub.2 O.sub.3 are incorporated to a greater or lesser degree of completeness into the Si.sub.3 N.sub.4 lattice, but the resulting mixed crystal exhibits a significantly worse behavior in response to temperature changes, which at least considerably limits its intended use.
It would therefore be obvious to produce a hot pressed Si.sub.3 N.sub.4 product which is at least substantially free of sinter-enhancing additives being incorporated therein. But this has not yet been possible when conventional pressing techniques are employed with graphite molds (G. R. Terwillinger, F. F. Lange, "Hot Pressing Behaviour of Si.sub.3 N.sub.4 ", J. Am. Cer. Soc. 57 (1974) page 25-29). In such techniques, Si.sub.3 N.sub.4 in the form of bulk powder or a blank is pressed monoaxially or pseudoisostatically in a graphite mold. Temperatures of 1600.degree.-1850.degree. C. and compression pressures of 10-35 MN/m.sup.2 (Mega Newton/m.sup.2) are used. The starting powder generally has a grain size of .ltoreq.10 and an impurity content of &gt;0.8%.
Another method is disclosed in German Offenlegungsschrift No. 2,536,676. According to this method, it is possible to produce Si.sub.3 N.sub.4 molded bodies which are free of alloys, but which are highly dense. However, this method is very complicated to use and, thus, this process has not been developed beyond the laboratory stage.
A need therefore exists to provide a method and apparatus for practicing the method wherein hot-pressed Si.sub.3 N.sub.4 bodies or preforms can be produced which are highly dense and alloy-free.
A significant object of the present invention is the provision of a method for producing hot-pressed Si.sub.3 N.sub.4 bodies or preforms that are highly dense and free of alloy contamination.
Another object of the present invention is an apparatus for carrying out the methods of the present invention.
Still another object of the present invention is the production of hot-pressed Si.sub.3 N.sub.4 bodies or preforms that exhibit good stability, high resistance to thermal shock, and a high temperature (HT) plasticity which is low.
A further object of the present invention is the use of pressing techniques employing a graphite vessel without any reaction taking place between said vessel and the silicon nitride.
A still further object of the present invention is the provision of a method that prevents the reaction of Si.sub.3 N.sub.4 with air or the humidity of the air so that optimum hot pressing of Si.sub.3 N.sub.4 can be achieved.